Move nostr peer rendezvous and mDNS into dedicated module homes

Relocate the overlay peer-rendezvous subsystem out of the overloaded
src/discovery/ tree into two focused, independent homes: src/nostr/
(relay-mediated overlay endpoint advertise/resolve/auto-mesh plus NAT
traversal) and src/mdns/ (link-local DNS-SD rendezvous). The two
subsystems are independent, so they get separate homes rather than
sharing one.

Drop the ambiguous "Discovery" stem from their identifiers in favor of
"Rendezvous": NostrDiscovery -> NostrRendezvous, LanDiscovery ->
LanRendezvous, and the matching config, policy, field, and method names.
The former src/discovery.rs handoff types (EstablishedTraversal,
BootstrapHandoffResult, the punch-packet helpers) fold into
src/nostr/handoff and stay reachable via the crate-root re-exports.

Pure relocation and rename: no logic, wire-format, config-key, metric,
or tracing-target changes. The operator-facing node.rendezvous.nostr.*
and node.rendezvous.lan.* config keys and the fips-overlay-v1 advert
namespace are byte-identical. cargo fmt/build/clippy clean; lib test
suite 1547 passing (baseline unchanged).

src/lib.rs

@@ -12,12 +12,13 @@ extern crate alloc;
 pub mod cache;
 pub mod config;
 pub mod control;
-pub mod discovery;
 #[cfg(target_os = "linux")]
 pub mod gateway;
 pub mod identity;
+pub mod mdns;
 pub mod node;
 pub mod noise;
+pub mod nostr;
 pub mod peer;
 pub mod perf_profile;
 pub(crate) mod proto;
@@ -39,8 +40,8 @@ pub use identity::{
 pub use config::{Config, ConfigError, IdentityConfig, NymConfig, TorConfig, UdpConfig};
 pub use upper::config::{DnsConfig, TunConfig};
 
-// Re-export discovery types
-pub use discovery::{BootstrapHandoffResult, EstablishedTraversal};
+// Re-export nostr rendezvous handoff types
+pub use nostr::{BootstrapHandoffResult, EstablishedTraversal, is_punch_packet};
 
 // Re-export tree types (relocated from tree:: to proto::stp)
 pub use proto::stp::{